From Phy.Org:
Li's team found that platelets release a molecule that suppresses the activity of cancer-fighting T cells. That molecule, unsurprisingly, was TGF-beta, which has been recognized for decades for its role in cancer growth. Yet this study is the first of its kind. Most TGF-beta is inactive. Li and his group found that the surface of platelets has a protein called GARP, a molecular hook that is uniquely able to trap and activate TGF-beta. Platelets, which are small cell fragments that circulate throughout the blood and are normally involved in clotting, become the major source of activated TGF-beta that invading tumor cells use to suppress T cells. In other words, platelets help give tumors their invisibility cloak from the immune system. Scientists have known for several years that certain cancers suppress T cells to avoid the immune system. That is why adoptive T cell therapy is one of the most promising advances in modern cancer treatment. It is a type of immunotherapy that awakens the immune system by retraining a patient's T cells to recognize their cancer. T cells are isolated from a patient's blood and retrained, or "primed," to recognize tumor cells. They are then injected back into the patient's bloodstream where they can now hunt and fight cancer.
More here.